1. Technical Field
The present disclosure relates to onboard diagnostics methods and systems for automotive vehicles, in particular related to diagnosing exhaust gas recirculation (EGR) systems.
2. Background Art
Vehicular systems that affect automotive emissions are provided with onboard diagnostics (OBD) to determine whether a fault in the system has occurred and to alert the vehicle operator to procure maintenance when such a fault has occurred. Proper operation of the EGR system impacts emission levels and so proper operation is determined via an OBD routine.
One OBD routine, called an intrusive monitor, is described in U.S. Pat. No. 6,257,214, commonly assigned and incorporated herein in its entirety. It is known that at a given speed-torque operating condition that manifold absolute pressure (MAP) is higher when EGR is added to the intake air than with no EGR. To determine system integrity, MAP data are collected at an operating condition at which EGR is provided at both the normal EGR setting and with the EGR valve closed. If MAP at the two settings is the same, or nearly so, a fault in the EGR system is indicated. Simultaneously, MAF data are collected and analyzed for the same purpose, as EGR displaced air in the intake manifold and reduces air induction, i.e., a lower MAF reading. During the period that the EGR valve is turned off to perform the diagnostic, engine emissions are temporarily impacted. Although the routine contributes only a modest increase in emissions, this can present a challenge when attempting to meet very low emission targets.
In response to a desire to have an EGR monitor which doesn't impact emissions, a non-intrusive monitor was developed and is disclosed in U.S. Pat. No. 6,850,834 B1, which is commonly assigned and incorporated herein in its entirely. In some non-intrusive monitors, engine sensor data, such as MAP and mass air flow (MAF) are collected at normally calibrated operating conditions, i.e., no interference in the normal operating conditions is imposed to facilitate the EGR OBD routine. The data are classified into ranges and cast in equations describing expected behavior. That is, as described above, EGR is known to impact MAP in a manner which can be modeled or estimated. A linear relationship between MAP and MAF with no EGR can be determined. Data collected at which EGR is being used is expected to deviate from the relationship with no EGR. If, however, data collected with EGR lies on the no EGR line, then a fault in the EGR system is determined.
The statement that MAF is substantially linearly related to MAP at a given engine speed and EGR rate ignores the effect that carbon canister purging and variable valve timing (VVT) have on this relationship. In particular, during purging of the carbon canister air and fuel vapor are supplied to the intake manifold at a location downstream of the MAF sensor. The purge air introduced into the intake manifold affects MAP, but is not measured by the MAF sensor, thereby confounding the effects that EGR has on MAP. At steady state, VVT presents no confounding of the linear MAF and MAP relationship. However, a rapid change in VVT causes a manifold filling delay such that the MAF and MAP do not track each other. An EGR OBD routine can be confounded by the transient caused by rapid VVT adjustments. In typical engine installations, (i.e., not hybrid electric vehicles, non-HEVs) there is sufficient operating time at which canister purge is not occurring so that sampling can take place. Also, in non-HEVs, VVT timing is not employed aggressively, meaning that highly retarded valve timings are not used often and the rate of change in valve timing is typically modest. Thus, the problem of delay was not found to substantially impair the accuracy of the EGR OBD routine in non-HEVs. Because an HEV has the capability of providing less than the full torque to the wheels by being supplemented by the electric motor and providing more than the desired torque with the extra used to generate electricity, more aggressive use of VVT can be accommodated, both in highly retarded timings and rapid rates of change in valve timing. Additionally, in HEVs, the internal combustion engine is operated for periods of time with periods of inactivity in between while the electric motor is providing propulsion. Consequently, the carbon canister is being purged almost all of the time that the engine operates to properly purge the canister. Due to these two factors, it has found that the non-intrusive EGR OBD monitor developed for a non-HEV, when applied to a HEV application, suffers from false detection of EGR system failures, which leads to unnecessary trips to obtain unneeded service and customer dissatisfaction. Thus, a robust EGR OBD monitor for a HEV application is desired.